Sanity check.

code/modules/mob/living/carbon/carbon.dm

@@ -956,3 +956,8 @@
 	if(mood)
 		if(mood.sanity < SANITY_UNSTABLE)
 			return TRUE
+
+/mob/living/carbon/transfer_ckey(mob/new_mob, send_signal = TRUE)
+	if(combatmode)
+		toggle_combat_mode(TRUE, TRUE)
+	return ..()

code/modules/mob/mob.dm

@@ -449,7 +449,8 @@
 	return
 
 /mob/proc/transfer_ckey(mob/new_mob, send_signal = TRUE)
-	if(!ckey)
+	if(!ckey || !new_mob)
+		CRASH("transfer_ckey() called [ckey ? "" : "on a ckey-less mob[new_mob ? "" : " and "]"][new_mob ? "" : "without a valid mob target"]!")
 		return FALSE
 	if(send_signal)
 		SEND_SIGNAL(src, COMSIG_MOB_KEY_CHANGE, new_mob, src)